A variety of techniques are available for providing visual displays of graphical or video images to a user. In many applications cathode ray tube type displays (CRTs), such as televisions and computer monitors produce images for viewing. Such devices suffer from several limitations. For example, CRTs are bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications.
Matrix addressable displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power. However, typical matrix addressable displays utilize screens that are several inches across. Such screens have limited use in head mounted applications or in applications where the display is intended to occupy only a small portion of a user's field of view. Such displays have been reduced in size, at the cost of increasingly difficult processing and limited resolution or brightness. Also, improving resolution of such displays typically requires a significant increase in complexity.
One approach to overcoming many limitations of conventional displays is a scanned beam display, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. As shown diagrammatically in FIG. 1, in one embodiment of a scanned beam display 40, a scanning source 42 outputs a scanned beam of light that is coupled to a viewer's eye 44 by a beam combiner 46. In some scanned displays, the scanning source 42 includes a scanner, such as scanning mirror or acousto-optic scanner, that scans a modulated light beam onto a viewer's retina. In other embodiments, the scanning source may include one or more light emitters that are rotated through an angular sweep.
The scanned light enters the eye 44 through the viewer's pupil 48 and is imaged onto the retina 59 by the cornea. In response to the scanned light the viewer perceives an image. In another embodiment, the scanned source 42 scans the modulated light beam onto a screen that the viewer observes. One example of such a scanner suitable for either type of display is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference.
In another embodiment, a micro-electromechanical (MEMs) device operates as the scanner. The MEMs scanner may be uniaxial or biaxial. A number of MEMs scanners are known. For example, one such scanner is described in U.S. Pat. No. 5,629,790 to Neukermanns et al., entitled MICROMACHINED TORSIONAL SCANNER.
Where such MEMs scanners are resonant devices, it may be difficult to synchronize the scanning frequency of the MEMs device to a desired frequency. For example, where the MEMs scanner scans a beam that is modulated according to the horizontal synchronization component of a video signal, incoming data is modulated at a line rate that may or may not match the resonant frequency of the MEMs scanner. Similarly, where the scanner is used as an input device, decoding electronics may utilize a line rate that differs from the resonant frequency of the MEMs scanner.
Returning to display applications, sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user's field of view and presents an image that occupies a region 43 of the user's field of view 45, as shown in FIG. 2A. The user can thus see both a displayed virtual image 47 and background information 49. If the background light is occluded, the viewer perceives only the virtual image 47, as shown in FIG. 2B.
One difficulty that may arise with such displays is raster pinch, as will now be explained with reference to FIGS. 3-5. As shown diagrammatically in FIG. 3, the scanning source 42 includes an optical source 50 that emits a beam 52 of modulated light. In this embodiment, the optical source 50 is an optical fiber that is driven by one or more light emitters, such as laser diodes (not shown). A lens 53 gathers and focuses the beam 52 so that the beam 52 strikes a turning mirror 54 and is directed toward a horizontal scanner 56. The horizontal scanner 56 is a mechanically resonant scanner that scans the beam 52 periodically in a sinusoidal fashion. The horizontally scanned beam then travels to a vertical scanner 58 that scans periodically to sweep the horizontally scanned beam vertically. For each angle of the beam 52 from the scanners 58, an exit pupil expander 62 converts the beam 52 into a set of beams 63. Eye coupling optics 60 collect the beams 63 and form a set of exit pupils 65. The exit pupils 65 together act as an expanded exit pupil for viewing by a viewer's eye 64. One such expander is described in U.S. Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporated herein by reference. One skilled in the art will recognize that, for differing applications, the exit pupil expander 62 may be omitted, may be replaced or supplemented by an eye tracking system, or may have a variety of structures, including diffractive or refractive designs. For example, the exit pupil expander 62 may be a planar or curved structure and may create any number or pattern of output beams in a variety of patterns. Also, although only three exit pupils are shown in FIG. 3, the number of pupils may be almost any number. For example, in some applications a 15 by 15 array may be suitable.
Returning to the description of scanning, as the beam scans through each successive location in the beam expander 62, the beam color and intensity is modulated in a fashion to be described below to form a respective pixel of an image. By properly controlling the color and intensity of the beam for each pixel location, the display 40 can produce the desired image.
Simplified versions of the respective waveforms of the vertical and horizontal scanners are shown in FIG. 4. In the plane 66 (FIG. 3), the beam traces the pattern 68 shown in FIG. 5. Though FIG. 5 shows only eleven lines of image, one skilled in the art will recognize that the number of lines in an actual display will typically be much larger than eleven. As can be seen by comparing the actual scan pattern 68 to a desired raster scan pattern 69, the actual scanned beam 68 is "pinched" at the outer edges of the beam expander 62. That is, in successive forward and reverse sweeps of the beam, the pixels near the edge of the scan pattern are unevenly spaced. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because the image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern 69, the displaced pixel locations can cause image distortion.
For a given refresh rate and a given wavelength, the number of pixels per line is determined in the structure of FIG. 3 by the mirror scan angle .theta. and mirror dimension D perpendicular to the axis of rotation. For high resolution, it is therefor desirable to have a large scan angle .theta. and a large mirror. However, larger mirrors and scan angles typically correspond to lower resonant frequencies. A lower resonant frequency provides fewer lines of display for a given period. Consequently, a large mirror and larger scan angle may produce unacceptable refresh rates.